Graphics displays such as LCDs are organized according to rows and columns. A pixel in the LCD display is addressed by activating a column driver and a row selector. Separate buffer amplifiers (column drivers) are employed to drive each respective column of the LCD. Thus, a typical LCD requires hundreds of buffer amplifiers to drive all of the columns in the display. Each of the buffer amplifiers is generally required to drive a rail-to-rail signal to the respective one of the columns in the LCD.
Color LCDs typically include multiple color planes (e.g., RGB). Each pixel address typically includes a separate pixel for each color plane. Pixels in the LCD are arranged as charge storage elements that are represented as capacitors. Each row selector operates as a switch that couples the output of a column driver to pixel in the LCD array. The charge stored in the pixel is an analog quantity that determines the brightness associated with the pixel. For color pixel arrays, the color associated with a selected pixel is determined by the charge stored in each of the pixels associated with the color planes. A typical color LCD also requires hundreds of buffer amplifiers to drive all of the columns in the display.
Pixels in the LCD are susceptible to damage when a DC voltage is maintained across the LCD for long periods of time. The liquid crystal damage is a result of charge migration across the liquid crystal, possibly de-ionizing the material. The result of the charge migration is that the LCD material will stick to the surfaces and cause image retention issues such as a sticking image. To prevent damaging the LCD material, the polarity of the signal applied to the LCD pixel is periodically reversed, typically every frame. An example LCD display system uses an alternating pixel pattern referred to as pixel inversion. In a pixel inversion system, each LCD column must be operated about a common voltage such that the output for each odd column is operated in an opposite range (e.g., from VDD to VDD/2) as the output for the even columns (e.g., from VDD/2 to VSS).
A liquid crystal display (LCD) system is illustrated in FIG. 1A. The LCD display system includes an LCD array that is organized according to rows and columns. A timing and control block receives video data and generates the necessary timing signals to selectively activate pixels in the LCD system. The timing and control signals activate a pixel by enabling a column driver and a row selector. Thin film transistor (TFT) type displays have a transistor array that is placed on top of liquid crystal array to operate as the row selectors.
The column drivers from FIG. 1A can be arranged as part of a charge share topology with the addition of extra switching circuits and capacitors as illustrated by FIG. 1B. N column drivers (DRV1-DRVN) include corresponding inputs IN1 through INN and outputs that are connected to column lines 1 through N. An array of switches (S1-SN) is used to connect all of the outputs of the column drivers. For example, just prior to the columns switching between voltage ranges (e.g., many LCD displays use line inversion, and often larger sized displays use pixel inversion) the switches are activated to couple the column outputs together to a charge storage capacitor (CSTORE).